The invention concerns a data medium with a printed image produced by the intaglio printing process, with adjacent ink areas with different ink layer thicknesses, an intaglio printing process for the printing of adjacent ink areas, as well as printing plates for carrying out the intaglio process and a process for the manufacture of the printing plates.
A characteristic of intaglio printing is that in the printing—that is, the colour transferring—areas, the surface material of a printing plate is removed by means of a suitable engraving tool or by etching. Ink is applied to the finished printing plate and the surplus ink is removed from the surface of the printing plate before the actual printing procedure by means of a doctor blade or a wiping cylinder, so that the ink remains only in the depressions. Then a substrate, usually paper, is pressed against the printing plate and then pulled off again, so that the ink remains adhering to the substrate surface and forms a print image there. If translucent inks are used, the thickness of the ink application determines the colour tone.
With previous gravure printing techniques, a distinction has been drawn between rotogravure and intaglio printing. In the case of rotogravure, the printing plates are made by means, for instance, of an electron beam, laser beam or graver. It is a characteristic of photogravure that different grey or colour scale values in the printed image are created by cells regularly arranged in the printing plate with varying density, size and/or depth.
The intaglio printing technique, and especially the steelplate intaglio printing technique, is an important technique for the printing of data media, especially securities such as bank notes and the like. In comparison with other common printing techniques, such as offset printing, for instance, the intaglio printing process allows a very thick ink deposition onto data media. The relatively thick ink layer generated in the intaglio process is readily recognisable to the lay person as a simple authenticity feature, due to its tactile quality. This authenticity feature cannot easily be reproduced with a simple copy, so that the intaglio printing technique offers protection against simple forgeries.
Intaglio printing is distinguished by the fact that linear depressions are formed in the printing plates in order to create a print image. In the case of the mechanically produced intaglio printing plate, due to the normally conical shape of the engraving tool, increased engraving depth produces a broader line. Furthermore, the ink capacity of the engraved line and thus the opacity of the printed line increases with increasing line depth. For the etching of intaglio printing plates, the non-printing areas of the plate are covered with a chemically inert lacquer. During the subsequent etching, the engraving is created in the exposed areas of the plate, such as the depth of width of the engraved lines depend in particular on the etching duration.
A process is known from WO 97/48555, with which intaglio printing plates can be produced in a reproducible, mechanical manner. To that end, the lines on a line original are recorded and the area of every line is determined exactly. With an engraving tool, for instance a rotating graver or a laser beam, firstly the outer contour of this area is engraved, to provide a clean outline around the area. Next, the outlined region of the area is cleared out with the same or another engraving tool, so that the entire line is exactly engraved according to the line original. Depending on the shape and movement of the engraving tool, on the base of the cleared area, a floor roughness pattern is formed, which serves as an ink trap for the printing ink.